Currently, passive synthetic aperture electromagnetic wave imaging technology has been widely used in the fields of radio astronomy, earth remote sensing and detection etc. Its frequency band covers entire microwave, millimeter wave and terahertz bands, and can continue to expand to electromagnetic wave with a higher frequency band as development of the technology. An imaging principle thereof is described as follows.
In particular, a plurality of receive antennas are arranged in a plane in space to form an antenna array. Such a plane is referred to as an antenna plane. Signals received by an i-th antenna and a j-th antenna in the antenna array are complex correlated to obtain a visibility value Vij. Assuming that the number of the receive antennas is N, N(N−1) visibility values may be obtained. In fact, depending on the arrangement of the receive antennas, there is redundancy in the N(N−1) visibility values, and a part of redundant visibility values may not be calculated. According to a passive synthetic aperture electromagnetic wave imaging algorithm, an angle distribution of electromagnetic wave emitted toward the antenna plane may be calculated.
When a target object is far enough away from the antenna plane (i.e., a size of the antenna array is less than, for example, one percent of a distance of the target object to the antenna plane, the size of the antenna array may be represented by a distance between two antennas which are spaced by a maximum distance), It can be considered that the angular distribution is a spatial distribution of intensity of the electromagnetic wave emitted around by the target object. Traditional fields to which passive synthetic aperture electromagnetic wave imaging technology is applied, such as the radio astronomy, earth remote sensing and detection etc. meet such a condition.
The visibility value required for the passive synthetic aperture electromagnetic wave imaging algorithm should be calculated by electromagnetic wave signals going through the antenna receive surface on the antenna plane. But actually, the visibility value may be obtained only after the received electromagnetic wave is transmitted and processed by the antennas and subsequent circuits, and gains of the respective antennas and the subsequent circuits are also different. Thus, it is required to correct the visibility value. In addition, it is required to correct the visibility value either, if a short-range (i.e., for example, the size of the antenna array is more than e.g. one percent of the distance from the target object to the antenna plane) target object needs to be imaged.
A feasible correction method is described below. In particular, Vij is used to V′ij represent an actually measured visibility value, i.e., a visibility value before correction, and is used to represent a visibility value required for the passive synthetic aperture electromagnetic wave imaging algorithm, i.e., a visibility value after correction. Assuming that a target object which is spaced from the antenna plane by L needs to be imaged, a point-like electromagnetic wave source may be placed at a position on an axis of the antenna array which is spaced from the antenna plane by L. The visibility value Vij of the i-th antenna and the j-th antenna may be measured, and a correction factor Cij=Vij. When imaging measurement is performed on the target object, V′ij=Vij/Cij. Then, V′ij is substituted into the passive synthetic aperture electromagnetic wave imaging algorithm to obtain a clear image of the target object which is near the axis of the antenna array and is spaced from the antenna plane by L. L is referred to as a focal length of the system.
However, the passive synthetic aperture electromagnetic wave imaging system using the above correction method can only clearly image the target object which is spaced from the antenna plane by L. When the distance between the target object and the antenna plane deviates from L, the image of the target object becomes blurred rapidly. Re-correction is thus necessary to change the focal length of the system to a desired value, in order to image an object at other distances. This causes it very inconvenient to use the passive synthetic aperture electromagnetic imaging system in imaging a short-range target, and even not possible to use the passive synthetic aperture electromagnetic imaging system in imaging a moving target.